1. Field of the Invention
The present invention relates to a driving technique for a light emitting element.
2. Description of the Related Art
In recent years, as a backlight of a liquid crystal panel or as an illumination device, a light emitting apparatus is employed which is configured using a light emitting element such as an LED (light emitting diode) or the like. FIG. 1 is a circuit diagram showing an example configuration of a light emitting apparatus according to a comparison technique investigated by the present inventor. A light emitting apparatus 1003 includes a single-channel LED string (light emitting element) 6 and a switching power supply 1004.
The light emitting element 6 includes multiple LEDs connected in series. The switching power supply 1004 is configured to step up the input voltage VIN input to an input terminal P1, and to supply a driving voltage VOUT to one end of the light emitting element 6 connected to an output terminal P2.
The switching power supply 1004 includes an output circuit 102 and a control IC (Integrated Circuit) 1100. The output circuit 102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 is configured to control the on/off time ratio (duty ratio) of the switching transistor M1 so as to adjust the driving voltage VOUT.
A switch (transistor) M2 for a PWM dimming control operation (which will also be referred to as a “burst dimming control operation”) and a first resistor R1 for current detection are provided on a path of the light emitting element 6. The control IC 1100 receives, from a host processor 9, a first pulse signal G1 used for the PWM dimming control operation and having a duty ratio adjusted according to the target luminance level of the light emitting element 6. The first driver 50 is configured to switch on and off the dimming switch M2 according to the first pulse signal G1.
A voltage drop (detection voltage) VR1 develops at the first resistor R1 in proportion to the driving current ILED that flows through the light emitting element 6. The error amplifier 10 is configured to amplify the difference between the detection voltage VR1 and a predetermined reference voltage VREF, so as to generate a feedback voltage VFB. The error amplifier 10 includes a transconductance (gm) amplifier 12 and a phase compensating circuit 14, for example. The phase compensating circuit 14 includes a capacitor CFB and a resistor RFB used for phase compensation.
An oscillator 20 is configured to generate a cyclic signal SOSC having a predetermined frequency. A pulse modulator 1030 is configured to operate in synchronization with the cyclic signal SOSC, and to generate a second pulse signal G2 pulse modulated according to the feedback voltage VFB. A second driver 40 includes an AND gate 42 and a main driver 44. In an off period in which the first pulse signal G1 is set to a first level (low level), the second driver 40 is configured to turn off the switching transistor M1. In an on period in which the first pulse signal G1 is set to a second level (high level), the second driver 40 is configured to switch on and off the switching transistor M1 according to a third pulse signal G3 that corresponds to the second pulse signal G2.
A second resistor R2 is provided in order to detect a current IL1 that flows through the inductor (coil) L1. During the on time of the switching transistor M1, a voltage drop VR2 develops across the second resistor R2 in proportion to the coil current IL1. The control IC 1100 is configured to prevent the voltage drop VR2 from exceeding a predetermined threshold value and thereby provides overcurrent protection. Furthermore, in a case in which the switching power supply 1004 is configured as a peak current mode DC/DC converter or an average current mode DC/DC converter, the duty ratio of the second pulse signal G2 is adjusted according to the voltage drop VR2.
FIGS. 2A and 2B are operation waveform diagrams each showing the light emitting apparatus 1003 shown in FIG. 1. FIG. 2A shows the operation of the pulse modulator 1030. The oscillator 20 is configured to oscillate with a predetermined frequency. The second pulse signal G2 is set to high level according to a positive edge of the cyclic signal SOSC, which turns on the switching transistor M1. When the switching transistor M1 is turned on, the coil current IL1 that flows through the inductor L1 rises with time. As a result, the voltage drop VR2 at the second resistor R2 rises in proportion to the coil current IL1. When the voltage drop VR2 reaches the feedback voltage VFB, i.e., when the coil current IL1 reaches a peak current IPEAK, the second pulse signal G2 is set to low level, which turns off the switching transistor M1. Subsequently, upon receiving a positive edge of the cyclic signal SOSC, the second pulse signal G2 is set to high level again, which turns on the switching transistor M1. The pulse modulator 1030 is configured to repeatedly perform the aforementioned operation.
FIG. 2B shows the PWM dimming control operation. The pulse modulator 1030 is configured to generate the second pulse signal G2. During the on period in which the first pulse signal G1 is set to high level, the first driver 50 is configured to turn on the dimming switch M2, and the second driver 40 is configured to switch on and off the switching transistor M1 according to the second pulse signal G2. During the off period in which the first pulse signal G1 is set to low level, the first driver 50 is configured to turn off the dimming switch M2, and the second driver 40 is configured to suspend the switching of the switching transistor M1.
With such a configuration described above, during the on period in which the dimming switch M2 is turned on, a feedback operation is performed such that the detection voltage VR1 matches the reference voltage VREF, thereby stabilizing the driving current ILED such that the relation ILED=VREF/R1 holds true. By changing the duty ratio of the switching of the dimming switch M2 using the driving current ILED thus stabilized as a reference voltage, such an arrangement is capable of changing the time average quantity of the current that flows through the light emitting element 6. Thus, such an arrangement allows the luminance level of the light emitting element 6 to be changed according to the duty ratio. Related techniques have been disclosed in Japanese Patent Application Laid Open No. 2009-261158, for example.
The present inventor has investigated such a light emitting apparatus including a single-channel LED string shown in FIG. 1, and has come to recognize the following problem. FIG. 3 is a waveform diagram showing a problem of the light emitting apparatus 1003 shown in FIG. 1. FIG. 3 shows the first pulse signal G1, the third pulse signal G3, the electric potential VLX at a connection node that connects the inductor L1 and the switching transistor M1, and the current ID that flows through the rectifier diode D1.
With the control IC 1100 shown in FIG. 1, the oscillator 20 operates in a free-running state. Thus, shown in FIG. 1, the cyclic signal SOSC and the first pulse signal G1 are generated non-synchronously. Thus, depending on the timing t1 at which the first pulse signal G1 transits from low level to high level, an irregular third pulse signal G3 is generated after the light emitting apparatus is turned on. Specifically, the off period TOFF1 of the switching transistor M1 is short immediately after the on period TON1 thereof.
The switching transistor M1 is turned on again (TON2) after such a short off period TOFF1, and, as indicated by the area encircled by a broken line (A) in FIG. 3, the back electromotive force VLX that develops at the node LX greatly swings in the negative direction. Thus, there is a need to configure each of the switching transistor M1, the inductor L1, and the rectifier diode D1 of the output circuit 102 as a high-voltage element having a high breakdown voltage that is equal to or higher than the back electromotive force VLX. This leads to a problem of a high cost and a problem of an increased circuit area.
It should be noted that such a phenomenon in which the back electromotive force VLX greatly swings in the negative direction immediately after the transition from the off period to the on period in the PWM dimming control operation, and the cause of this phenomenon, have been uniquely studied by the present inventors, and are by no means within the scope of common and general knowledge of those skilled in this art.
Also, such a problem not only occurs in the light emitting apparatus 1003 shown in FIG. 1, but can also occur in a light emitting apparatus including a constant current driver instead of the first resistor R1.